We are studying the changes in membrane properties of chick embryonic myocardial cells which occur during development and in cell culture. There are changes in electrical properties of the resting membrane, notably in K ion conductance, and of the active membrane, notably a change from tetrodotoxin (TTX)-insensitive slow Na ion channels to TTX-sensitive fast Na ions channels, during development. The specific activity of the (Na, K ions)-ATPase also increases during development. In culture, these properties rapidly revert back to the early embryonic state. We plan to study the time course of this reversion, and to determine the influence of the innervations (and autonomic agents) on these changes. To further explore the role of innervations, we have begun a program of organ cultured hearts from chick embryos of various ages, and have begun combined nerve and muscle cell cultures. We will examine the slow Ca ions channels, and the influence of catecholamines and other agents at different stages of development. We will examine the binding of H3-ouabain, H3-TTX, and H3- epinephrine as a function of embryonic age. We will attempt voltage clamping the cells at different stages of development and isolated single cells in culture, and do cation flux measurements for passive and active movements. The role of Ca ions current across the sarcolemma and transverse (T) tubule walls and of the SR tubules in electromechanical coupling will be studied in chick hearts and in the hearts of mammals and other animals. Correlative cytochemistry (e.g., localization of intracellular Ca ions, (Na, K ions)-ATPase, (Ca, Mg ions-)-ATPase) and electron microscopy is being done in normal and abnormal hearts. We are doing morphological studies on the development of T tubular system. We plan to continue studies on the (Na, K ions)-ATPase and to examine the activity of the (Ca, Mg ions)-ATPase (microsomal fraction) during development and in normal and pathological states. Ultrastructural changes in the intercalated disks and gap junctions are being examined under a variety of conditions and correlated with electrophysiological changes. Experimental and theoretical studies on the function of myocardial cell junctions will be continued.